Delays in language development and impairments in communication ability constitute a diagnostic feature of Autistic Disorder and PDD-NOS/Atypical Autism; communication challenges are also well documented among children with Asperger's Disorder. The goal of this project is to characterize the early language development of young children diagnosed with one of these pervasive developmental disorders (who will be referred to as children with an ASD). Our starting point is the empirical overlap recently demonstrated between the language impairments observed within the autism spectrum and those observed outside it (in language specific disorders), and our theoretical motivation is whether the language impairments observed in ASD involve a unique constellation of phenomena (the distinct category account), or whether they differ only quantitatively from those observed in language-specific disorders (the dimensional account). Our first specific aim will characterize the early language skills of a large sample (N=170) of ASD children at age 2;6; a parallel specific aim will evaluate language outcomes by assessing the same children at age 5;6 (Studies 1 and 8). Further specific aims will compare longitudinally two ASD subgroups ("late talker" and "normal vocabulary," N=74, 37 per subgroup) with two recently collected samples - one of late talkers with specific language delay and one of typically developing young children - who received the same assessments as those proposed for the ASD participants (Studies 2-7). This longitudinal comparison will utilize a series of group design experimental studies, as well as microgenetic methods, to examine mechanisms and patterns of novel word learning, the link between lexical and grammatical skills, and the relation between early language abilities and verbal repetition. Findings from this project will significantly advance our understanding of the nature of early language development in ASD, will lead to more accurate early diagnosis by clarifying the boundaries between late talkers with and without ASD, and will provide more refined language phenotypes that should direct better fitting treatment approaches.